On Wed, Jul 29, 2009 at 10:08 AM, Philip Lord<random6000@mac.com> wrote:
> Thanks guys,

You bet.

> It's going to take me a little time to go through all this stuff. I did
> however have a few moments to quickly check the voltages across pins 1 & 4
> and 5 & 4 of the J8 power connector (disconnected from motherboard)...they
> both read about 8.2V AC, which unfortunately isn't 12V as Ruud mentioned.

I don't have a static PET to check (anymore), but 8.2VAC is at least
close.  The critical thing is that the 7805 needs some "headroom" for
it to properly regulate.  ISTR that 7.5VDC is close to the minimum
input voltage for a true 7805 to work (there are plenty of "low
dropout" regulators now that will work with the input under 6VDC).

Check the input DC to the regulators with the board plugged into the
PSU (well... the transformer and filter cap ;-) and check that against
the datasheet for the 7805 if it happens to be below  8VDC.  You could
be experiencing a problem with the rectifier (the diodes between the
transformer and the 7805s) or the harness connector (I've seen some
charred ones in the past from too much resistance leading to too much
heat, etc).

> I'll update you both (and the list) once I have had more time for testing,
> starting with the 7805's

Sure.  With problems like this, check all your power and work
backwards.  At some point, the outputs and the inputs won't match and
your fault is between the two.

-ethan

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